1. Field of the Invention
The present invention is related to free space optical systems and, in particular, to a multiple function detector in a free space optical system.
2. Background Information
With the increasing popularity of wide area networks, such as the Internet and/or World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions is the use of wireless optical telecommunications technology. Wireless optical telecommunications utilize beams of light, such as lasers, as the carriers of communications signals, and therefore do not require the routing of cables or fibers between locations. Data or information is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver. The receiver includes a communications detector (including a demodulator or decoder) to extract the data or information from the optical signals.
For point-to-point free space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of a transmitter and of the receiver properly pointed at each other. For example, a transmitted optical beam with a one milli-radian divergence has a spot diameter at the receiver of about one meter at a one-kilometer range. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field-of-view) could compromise the link unless active tracking is employed.
The transmitters and receivers are typically located in optical terminals, which may be located in windows of buildings, on rooftops, or other architectural structures. xe2x80x9cTrackingxe2x80x9d is considered from the receiving terminal""s perspective and refers to the ability of the receiving terminal to see the beam and to focus the beam onto the receiving terminal""s detector. The receiving terminal tracks the received beam""s direction in two angular degrees of freedom.
During communications, both terminals are transmitting to each other and receiving from each other. In this scenario, proper beam tracking can be difficult, especially considering the fact that the target at the receiving terminal may be quite small relative to the large separation distance between the transmitting and receiving terminals. Additionally, because buildings (or other architectural structures where terminals may be located) vibrate and sway with wind and temperature changes the receiving terminal is usually moving with respect to the transmitting terminal.
There are several known ways to control tracking. For example, in one known system, a free space optical system receiver receives an incoming light beam from a transmitting terminal. A lens or other collection optics collects light from the light beam and focuses it onto a beam splitter, which directs a major portion of the light beam to a communication detector. The communication detector recovers the wideband digital information carried on the light beam. In addition, the beam splitter directs a small portion of the light beam to a tracking detector. The tracking detector detects when the free space optical receiver and the transmitting terminal are misaligned. This tracking information is used to adjust the alignment between the receiver and the transmitting terminal.
In an effort to design and build free space optical systems, smaller, fewer and/or cheaper components may prove beneficial. Although devices to implement the communication detector are commonly low cost devices, such as a PIN photodiode, or an avalanche photodiode (APD), devices to implement the beam splitter and the tracking detector are commonly quite expensive. As such, any free space optical system that could eliminate such expensive components would be advantageous. Additionally, any free space optical system that has fewer components may be cheaper to build and operate because both the major portion of the light beam and the small portion of the light beam have to be aligned to the communication detector and the tracking detector, respectively.
In accordance with aspects of the present invention, an optical tracking system for use in an optical receiver of an optical communication system is provided. In one aspect, the optical tracking system includes an optical fiber assembly. In addition to an optical fiber, the optical fiber assembly includes a ferrule with a face having several facets and/or optical element coatings. These facets and optical element coatings reflect and/or diffract a portion of a received free space optical signal, the distribution of the reflected or diffracted light depending on the alignment between the optical receiver and the received free space optical signal. The optical tracking system also includes one or more tracking detectors to detect the reflected or diffracted light and generate signals as a function of the distribution of the reflected or diffracted light. These signals can be used to adjust the alignment between the optical receiver and the received free space optical signal. The optical fiber is used to propagate the remaining portion of the received free space optical signal to a communications detector.
In a further refinement, the optical assembly may include collection optics to help direct the reflected or diffracted light to the tracking detectors.
In another aspect, the optical fiber assembly includes a quadrant cell and an optical fiber. The optical fiber is inserted in an opening in the center of the quadrant cell. The optical fiber is used to propagate light from the received free space optical signal that is incident on the fiber to a communications detector. The quadrant cell is used to detect light (if any) of the received free space optical signal that misses the optical fiber. The quadrant detector generates signals as a function of the distribution of the light that misses the optical fiber. These signals can be used to adjust the alignment between the optical receiver and the received free space optical signal.
In still another aspect, the optical fiber assembly includes a mounting plate and an optical fiber mounted in the center thereof. The mounting plate is grooved and several tracking fibers are positioned in the grooves. The optical fiber is used to propagate light from the received free space optical signal that is incident on the fiber to a communications detector. The tracking fibers used to detect light (if any) of the received free space optical signal that misses the optical fiber. The tracking fibers propagate signals as a function of the distribution of the light that misses the optical fiber. These signals can be used to adjust the alignment between the optical receiver and the received free space optical signal.
In yet another aspect, the optical fiber assembly includes a central optical fiber encircled with several tracking fibers. As in the mounting plate aspect, the optical fiber propagates light from a received free space optical signal to a communications detector, whereas the tracking fibers propagate signals as a function of the distribution of the light that misses the optical fiber. These signals can be used to adjust the alignment of the optical receiver and the free space optical signal.